Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A diode typically includes a PN junction. The P-type material forms an anode, and the N-type material forms a cathode. At the junction between the two materials, electrons from the N-type material diffuse into the P-type material, and leave behind positively charged donor ions on the N-type side of the junction. Similarly, holes from the P-type material diffuse into the N-type material, and leave behind negatively charged ions on the P-type side of the junction. The electrons and holes eventually recombine in the P-type material, and N-type material, respectively, but the positive and negative charges on either side of the junction remain. The charges therefore electrically polarize the junction and create an electric field that depletes the region surrounding the junction of free charge carriers, at which point further diffusion of electrons and holes ceases. The electrical polarization also defines a potential barrier at the junction that opposes external current. But when a voltage is applied to positively charge the anode relative to the cathode, the potential barrier of the depletion region can be overcome by the external voltage and external current can flow from the anode to the cathode via recombination events in the vicinity of the depletion region.
A photodiode is a type of diode in which the depletion region is a light-sensitive region where incident photons generate electron-hole pairs via the photoelectric effect. The electrical polarization of the depletion region causes the generated electron-hole pairs to separate and diffuse in different directions. Electrons diffuse toward the accumulation of positive charge adjacent the depletion region in the N-type cathode, and holes diffuse toward the accumulation of negative charge adjacent the depletion region in the P-type anode. The internal generation and diffusion of electron-hole pairs creates a reverse photocurrent within the photodiode (from the cathode to the anode) that is linearly proportionate to the flux of incident light (and thus the rate of electron-hole pair generation). When the photodiode is reverse biased (with the cathode at a greater voltage than the anode), the electrical polarization of the depletion region is reinforced by the reverse bias and the depletion region becomes larger, and more sensitive to photocurrent generation. Moreover, the externally applied reverse bias voltage provides a current sink to drain current to and a current source to draw current from and thereby allow the photodiode to output the generated reverse photocurrent. While the photodiode is reverse biased and generates a reverse photocurrent linearly proportionate to the flux of incident light, the photodiode is operating in photoconductive mode.
When the photodiode is not reverse biased, such as when the anode and/or cathode is allowed to float, the photodiode does not operate in photoconductive mode. The generation of electron-hole pairs still occurs in response to incident light, but without an applied reverse bias, the light-sensitive depletion region is not as large and the electrical polarization of the depletion region is not as strong. Moreover, with one terminal of the photodiode floating, an internally generated, ongoing photocurrent from the anode/cathode is not maintained. As a result, when the photodiode is not reverse biased, the generated electron-hole pairs do not create a reverse photocurrent proportionate to the incident light. Instead, an internal reverse photocurrent is generated due to the internal polarization of the depletion region. The internal reverse photocurrent causes charge to accumulate on the cathode and the anode, with the P-type anode becoming positively charged relative to the N-type cathode. Once the anode and cathode accumulate enough charge to counter the internal potential barrier of the depletion region, the photodiode reaches its open circuit voltage, at which point the internal current ceases. The initial photocurrent before the internal current ceases leaves the anode at a greater voltage than the cathode. The value of the voltage difference between the anode and the cathode (i.e., the open circuit voltage) scales with the logarithm of the flux of incident light. While the photodiode is not reverse biased and generates a voltage proportionate to the logarithm of the flux of incident light, the photodiode is operating in photovoltaic mode.